This invention pertains generally to inertial rate sensors such as rotation rate sensors and, more particularly, to a vibratory tuning fork for use in an inertial rate sensor.
As illustrated in FIG. 1, vibratory quartz tuning forks heretofore provided for use in inertial rate systems have included a pair of elongated tines 11 of constant rectangular cross section joined together at a crotch 12 which consists of a central segment 13 that is perpendicular to the axis of the device and one or more pairs of additional segments 14 which interconnect the first pair of segments and the tines. One example of such a tuning fork is found in European Patent Application No. EP 0 821 481 A2. A double ended tuning fork with straight tines of constant rectangular cross section is shown in U.S. Pat. No. 5,366,194.
In these tuning forks, the side faces 16 of the tines are parallel to each other throughout the length of the tines, as are the top and bottom faces 17. A tuning fork having this geometry has two fundamental lateral modes of vibration, one designated by the arrows 18 parallel to the plane of the fork, the other designated by the arrows 19 perpendicular to the plane of the fork. When the fork is resonating in either mode, the maximum stress on any face is generated at the base of face, and the minimum is generated at the tip. The distribution of the stress along the face is not linear, but rather is concentrated near the base, so that most of the outer portion of the face is at a low stress level. Furthermore, the local stress level in the corners where the crotch joins the tines is even higher due to the geometry of the fork and faceting which tends to occur with that geometry. Ignoring that concentration, the distribution of stress along the tines is approximately as follows:
Since quartz is piezoelectric, the stress at the face of the tine results in charge being developed on the surface, which can be collected by an electrode. Conversely, stress can be created by applying an electrical potential to the electrode. The magnitude of the charge produced is proportional to the integral of the stress and the area of the face.
This standard tine geometry has several disadvantages which limit performance of the tine. Due to manufacturing inaccuracies and faceting which can occur when a crystalline material is chemically etched, the cross sections of most tines are not truly rectangular. These errors cause the fundamental vibration mode to be aligned at an angle to the plane of the fork, rather than being aligned precisely with it. The facets will often be the largest in the corners between the crotch and the tines, as indicated at 21, where the stress is the highest and the facets have the greatest effect on the angle of tine deflection.
Also, with most of the charge being developed near the base of the tines, electrodes which extend along the length of the tines are inefficient. The outer portions of the electrodes are largely ineffectual due to the low stress levels toward the tips of the tines.
Another problem with the standard tine geometry is the high stress level at the junction of the crotch and the tines, which is increased even further by the stress concentration at that point. In order for the fork not to fracture, the maximum stress at the junction must be kept below the fracture strength of the fork material. Since the stress along the tines is considerably less than the level near the crotch, the charge available from the electrodes is also severely limited.
Moreover, the design of the tines is somewhat limited in flexibility, due to the fixed relationships between length, width, thickness and frequency. Thus, for example, given certain thickness and frequency requirements, only the length and width can be varied. Since frequency is proportional to the relationship of length and width, the design of the fork is limited to only a few variations. This permits very limited optimization of performance.
The central segment 13 in the crotch section of prior art tuning forks presents a further problem in that undesirable bridging facets 22 tend to form between that segment and the segments 14 next to it. Such facets tend to span an entire crotch segment or face. In addition, the presence of the central section limits the length of the other segments, which also tends to cause bridging.
It is in general an object of the invention to provide a new and improved tuning fork for use in an inertial rate sensor.
Another object of the invention is to provide a tuning fork of the above character which overcomes the limitations and disadvantages of tuning forks heretofore provided.
These and other objects are achieved in accordance with the invention by providing a tuning fork which has a body of piezoelectric material with a base and a pair of tapered tines which extend from the base and decrease in lateral dimension from the base toward the tips of the tines. A crotch section at the base of the tines has an inner pair of faces which lie in crystallographic planes of the piezoelectric material and an outer pair of faces which lie in additional crystallographic planes of the material and extend between the inner pair of faces and the tines. An isolation slot is formed in the base adjacent to the crotch, with enlarged cut-outs at the ends of the slot to prevent the formation of facets and membranes across the slot.